1. Field of the Invention
The present invention relates to a raw material alloy used in the production of an R-T-B system sintered magnet, an R-T-B system sintered magnet and a production method thereof. Here, R represents one or more elements selected from rare earth elements, and T represents one or more elements selected from transition metal elements and comprises Fe, or Fe and Co.
2. Description of the Related Art
The basic production process of an R-T-B system sintered magnet comprises: preparation of the raw material alloy; crushing of the obtained raw material alloy; compacting the crushed alloy powder in a magnetic field; and sintering and aging. Various approaches in the respective production steps have been attempted to improve the magnetic properties of an R-T-B system sintered magnet. For example, the approaches include lowering the atmospheric oxygen content in the production process to lower the oxygen content in the sintered body, using multiple (normally two) raw material alloys and the like. As will be described below, among such approaches investigations are being conducted into improving magnetic properties by improving the microstructure of the raw mother alloy.
Raw material alloys have conventionally been prepared using metal mold casting, that is, ingot casting and strip casting which uses a cooling roll to quench a molten alloy.
The formation of α-Fe cannot be avoided in an alloy prepared by an ingot method, which causes the crushing efficiency of the alloy to dramatically decline, whereby the magnetic properties of the ultimately obtained magnet are poor. To overcome this problem, it is known that α-Fe can be eliminated by solution-treating the alloy obtained in the ingot method. However, carrying out solution-treating can cause a drop in productivity and an increase in production costs.
On the other hand, α-Fe hardly forms at all in an alloy prepared by strip casting (e.g. Japanese Patent Laid-Open No. 5-222488 (Patent Document 1) and Japanese Patent Laid-Open No. 5-295490 (Patent Document 2)), which is one kind of rapid solidification. The resulting alloy has a grain size from 20 to 30 μm along the minor axis direction and up to about 300 μm along the major axis direction, meaning that a comparatively fine microstructure is formed.